1. Field of the Invention
This invention relates to devices used for the sudden release of energy and in particular it relates to seismic sound sources such as air guns, used to release a pressurized gas such as air, into a body of water.
2. Technical Description of the Prior Art
Conventional air guns contain a gas under pressure, usually air compressed to 500 to 8000 lb/in.sup.2 (3.448.times.10.sup.6 to 55.16.times.10.sup.6 Newtons/m.sup.2). A value mechanism is mechanically, pneumatically, or hydraulically actuated to suddenly release the compressed air over a period of a few milliseconds to generate an acoustic impulse.
In general, a conventional air gun has at least two chambers; a control chamber and a firing chamber. Usually, the two chambers are in fluid communication with each other via restricted metering passageways so that the air pressure can be equalized between the two chambers. The firing chamber is provided with exhaust ports which, when the gun is armed, are sealed by a valve such as a sliding sleeve, piston, or plunger valve. The valves, of whatever type, have two faces of different areas. The valve face exposed to the air pressure in the control chamber has the greater area. Accordingly, the differential forces acting against the two valve faces, hold the valve closed to seal the exhaust ports. To fire the gun, the differential forces are upset by lowering the pressure in the control chamber, or by slightly raising the pressure in the firing chamber. The pressure in the firing chamber forces the valve to abruptly open so that the air in the firing chamber explosively escapes through the exhaust ports. To open the exhaust ports, the valve moves in one direction only. Moving in the opposite direction, the valve closes the exhaust ports.
As noted above, valve members used in air guns may be of the sleeve, plunger, or piston type. Sleeve or plunger valve-members are moved by an actuator or driving mechanism against a seat to block the escape of high pressure air through the exhaust ports. To fire the gun the actuator then moves the valve member abruptly away from the seat in a direction parallel to the plane of the exhaust port. In the piston type, the valve member moves perpendicular to the plane of the exhaust port. In all types, the actuating or drive mechanism drives the valve member in one directon of motion to unblock the exhaust ports. The actuator then reverses the direction of motion of the valve member to drive it back against the seat so that the firing chamber can be recharged. Valve actuation systems must control large forces and typically require several complex pilot-valve amplification stages to perform their task.
U.S. Pat. No. 3,638,752 to Wakefield shows an air gun using a cylindrical sliding sleeve valve that abuts against a seat to close the exhaust ports. U.S. Pat. No. 3,039,292 to Ottestad reveals a plunger-type valve member. Typical piston type valves are shown in U.S. Pat. Nos. 3,276,534 to Ewing, 3,310,128 to Chelminski, and 3,896,898 to Kirby.
The two major disadvantages of the conventional, above-described guns are related to the speed of operation and to the complexity of the valve actuator system or driving mechanism that is required to control the large forces involved.
The prior-art air guns were wasteful of air because of their low speed of operation. The requirement for valve-member motion-reversal, that is, motion in a first direction to open the valve and motion in the reverse direction to close the valve, created an unavoidable lengthy period of time during which the exhaust ports remained open. Of course, compressed air bled from the firing chamber throughout the entire period that the valve was open during a firing cycle. It is known that it is only the initial outrush of air, which takes place just as the valve member first opens the exhaust ports, that generates useful acoustic energy. Further release of air following the initial outrush of air, such as in the prior-art guns contributes nothing to the strength of the acoustic pulse and is wasteful.
Another type of sound source, generally used to produce either continuous-wave or chirp signals, employ a type of rotary valve that acts as a siren. For example see U.S. Pat. Nos. 4,033,429 and 3,738,446. Siren type sound sources are not suitable for use where a single acoustic pulse is desired.
In related co-pending application Ser. No. 905,491, filed May 12, 1978, now U.S. Pat. No. 4,180,139, assigned to the assignee of this invention, an economical efficient seismic sound source is disclosed. Having a minimum of moving parts, this air gun includes a housing for containing the compressed gas and an exhaust port through which the gas may be released. A valve member or sealing pad is mounted for linear, back-and-forth motion in the chamber and may be positioned against the exhaust port in several sealing positions to block the port. The valve member has an orifice therethrough. The valve member may be moved by an actuator or shuttle in one continuous motion, without reversing direction, from one sealing position, through a firing position wherein the orifice is aligned with the exhaust port to emit an air jet, thence to another sealing position.
To fire the gun again, the valve member may be returned to the first sealing position. For additional firings the valve member may be driven back and forth between the two positions in alternate strokes, resulting in one firing for each stroke.
In operation of this air gun, the actuator or shuttle was driven back and forth in the housing by applying control-air pressure against the shuttle alternately at opposite ends of the gun. Two control-air hoses as well as a firing-air hose were required, extending from a boat to the gun. The resulting hose bundle was difficult to handle in the field. Furthermore, due to the great length of the hoses that were several tens to several hundreds of feet long, there was a lag between the time that air pressure was initially applied to the control-air line at the boat control panel, and the time that the gun actually fired. This invention is an improvement in the gun fire-control means.